The use of polymers as binders and wet proofing agents in cell electrodes containing finely divided electrochemically active material has been described and used for many years. In general, efforts have been made to improve the materials so that a minimum amount of binder can provide a maximum bonding effect for the active particles. Fibrous polymers have been proposed for this purpose. In U.S. Pat. Nos. 3,630,781 and 3,954,501, for example, electrodes are prepared from a fluorocarbon polymer very finely dispersed in a liquid, e.g., water. The dispersion, which is in essence a liquid-type dispersion, is mixed with a surfactant and finely divided active material, and the mixture is used in a paste or a liquid carrier, preferably on the colloidal level. At a stage in the process, the fluorocarbon is precipitated as a fibrous material, e.g., by freezing, increasing the pH, or by solvent extraction. Existing processes for converting polymers to fibrous form from such liquid-type dispersion have many problems. One of the most serious limitation of existing processes employing liquid dispersions is removal of the liquid in which the polymer is dispersed. This requirement introduces an energy intensive step which limits and complicates the processing of the material. Dispersing agents as usually formulated may contain a dispersing additive such as a surfactant, and such additives may have a detrimental effect on the performance of an electrode. It also may be difficult to control or obtain a desired uniform distribution of active material throughout the dispersion during processing. The liquid dispersion system also has constraints with respect to processing flexibility in fabricating the end product. For example, it is difficult to produce a membrane via a milling process using a liquid or paste. A liquid or paste must be applied to a support in order to be fabricated into a shaped product. Also, the polymeric binders prepared from liquid-type formulations limit the exposure of the active material, which limits the capacity of electrodes made from such materials.
A further problem is that the active particles may undergo changes in volume during the use, and it has been difficult to find a suitable polymeric electrode active material which will tolerate large changes in volume of the active material, should they occur in operation of the cell.
Co-pending U.S. application Ser. No. 226,502, filed Jan. 19, 1981 now U.S. Pat. No. 4,320,184, discloses an electrode and a method of preparing such electrode in which electrode active particles are held by a fibrillated polymeric material without the necessity of sintering the material or using a separate supporting structure. The method avoids the problems arising out of using liquid-type dispersions or emulsions to fabricate the electrode, and it makes possible greater flexibility in cell design. The present invention is a further improvement in that it gives a further dimension in freedom in cell design when needed.